Graphite Epoxy Motor Case Impact Monitoring and Reference Strain Measurement Using Conventional Resistance Strain Gages
AEROSPACE CORP EL SEGUNDO CA
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A series of low-energy impacts below damage threshold was monitored on subject graphite epoxy motor cases using conventional strain gages. Similar impact tests were done on an empty case and a Delta GEM inert-propellant-loaded case. The conventional strain gages were also used to monitor case bending during lifting, handling, and transportation shock exercises. The impact test energies ranged between 1 and 6 ft-lb with a 58-in. tup. The 6 ft-lb impacts impart a sufficiently large strain wave to be sensed by all gages on the inert loaded motor, regardless of location or orientation. This includes impacts on the cork covering. However, the strain signals for some of the remote impacts are pushing the lower limit of detection for this system 1 micro-strain. Typical strains measured for a 6 ft-lb impact on the inert-loaded case were 15 micro-strain where the gage was located 17 in. from the impact. The strain impulse damping with distance is roughly exponential for the sharp impulse and very slight for the lower-frequency ringing. The strain waves have remarkably reproducible amplitudes and shapes for successive impacts at a given location and energy. However, the recorded waves are dependent on factors such as the orientation of the gage with respect to the case axial vs. circumferential, the mode and direction of the wave propagation, and the location of the impact. These factors make it difficult to predict the exact wave shape for an arbitrary combination of impact location and gage orientation. The lifting exercises produced strain signals of 200 to 500 micro-strain, which are much larger than the impact strain signals. The strapping of the motor to the truck bed also imposed a significant strain offset 80 micro-strain to the dynamic signal monitoring. Running the truck over a series of wooden 2 x 4s produced dynamic strain oscillations of - 100 micro-strain with a nominal bandwidth of 10 Hz.
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